Fixes bug #6536. When MyODBC or any other client do my_init()/my_end()

several times, it was causing corruption of charset data stored in once_mem_pool. 
my_end() deallocated once_mem pool, but did not put a flag that charsets have 
to be reloaded. The fix addresses this problem.

mysys/my_init.c

@@ -152,6 +152,7 @@ void my_end(int infoflag)
       DBUG_PRINT("error",("%s",errbuff[0]));
     }
   }
+  free_charsets();
   my_once_free();
 
   if ((infoflag & MY_GIVE_INFO) || print_info)